The present invention generally relates to electrical systems which employ current transformers. More particularly the present invention relates to current transformers employed as circuit protection devices.
In some electrical distribution systems a zone protection system may be used to assure that electrical failures of a portion of a system may be isolated. For example, in an aircraft a protection zone or fault zone may be established between a generator and one or more electrical loads that may receive current from the generator. In a typical three-phase system, first current transformers may be placed around output conductors at the generator. Second current transformers may be placed on the conductors at a position remote from the generator. A monitoring system may detect any current imbalance between the first and second current transformers if and when a fault may develop in a zone between the first and second current transformers.
Secondary windings of the current transformers may be interconnected to a control circuit which may operate disconnection contactors in the event of a fault. In this context, the secondary windings may be considered to be connected to a load with low impedance. The current transformer design and the impedance of the control circuit may be selected so that, in normal operation, the voltage developed across the secondary windings does not exceed insulation breakdown limits of the secondary windings.
It is possible that a so-called “open circuit” failure may occur in the control circuit or in an interconnection path between one of the current transformers and the control circuit. In such an event, the impedance across the secondary winding may become infinite. Continued passage of current from the generator to the electrical loads may then produce extremely high voltages across the secondary winding. As a result of such high voltages, insulation in the secondary winding may break down and the current transformer may become inoperative.
In a typical aircraft generator, current transformers may be incorporated directly in the generator. Consequently, failure of one of the current transformers in the generator may result in a requirement to remove the generator from the aircraft to replace the defective current transformer.
As can be seen, there is a need to provide protection of current transformers from damage resulting from open circuit failures in a control circuit to which the secondary winding of the current transformer may be connected. Additionally, there is a need to avoid a need to remove and repair an aircraft generator in the event of such an open failure.